Endless Facts, One Snippet at a Time
What Determines a Baby’s Sex?
- The baby’s sex depends on which chromosome the sperm carries — X or Y.
- An X sperm makes a girl (XX), while a Y sperm makes a boy (XY).
- The egg always provides an X chromosome.
- Sperm carrying Y chromosomes move slightly faster but live shorter lives.
- Sex is determined at fertilization, long before birth.
- The presence of the Y chromosome triggers male development.
- Hormones guide the formation of reproductive organs in the embryo.
- External sex traits form by the end of the first trimester.
- Ultrasounds reveal the baby’s sex around 18–20 weeks.
- Genetics — not timing or diet — decides the outcome.